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I got a clear plastic tube (1-2 feet long) and pushed it onto the pointy fill cap (cut open the end first) of the oil bottle. Inserted the end of tubing into fill hole, raised the oil bottle as high as possible and squeezed the oil in till it ran out the fill hole. Duct tape the tube to the fill cap so it doesn't pop off.
usually at the auto parts stores they have a hand pump with a 1-2 foot clear plastic tube. the top of the pump screws right onto the 75-90w gear oil bottles. I bought one at kragen for $10 or so. I haven't changed the s2000 diff fluid, but I have on my 4runner a few times and it works very well. xviper's tube method or the syringe will both work as well.
I did the same as xviper - it was cheap, easy and not messy. This method also works when changing the transmission oil (except I slid the tubing onto a funnel, as the Honda MTF is sold in regular 1-qt containers).
I just go to Fisher Tire in Knoxville. It's easier this way, and I stand right there and watch the guy in the pit. They are very good people, and I don't have to worry about the disposal of fluids if I have them do it, which is an inconvenience in itself. Up to this point, I haven't let the car out of my site if I'm having anything done to it.
The pumps cost less than $5 and are made for the job, but then again, being creative can be fun too (just remember diff oil stinks so you don't want to be risking a spill).